1. Field of the Invention
The present invention relates to a node device for a ring system and a system and method for constructing multiple spanning trees by using the node device and, more particularly, to a method of constructing multiple spanning trees in such a manner that a transmission path in the form of a ring connecting a plurality of node devices is divided into multiple spanning trees, which are a plurality of logic topologies.
2. Description of the Prior Art
A spanning tree has been used to construct local area networks (LAN). Details of the spanning tree are specified in IEEE802.1D. In a case where a plurality of LANs are interconnected by abridge, if a loop is formed when a broadcast packet is send to the LANs, a phenomenon occurs in which passing of the broadcast packet through the loop is endlessly continued and the broadcast packet does not disappear. For the purpose of preventing such a phenomenon, a point in the communication path connected in loop form is logically disconnected by processing under a protocol called a spanning tree protocol (bridge protocol) to form a tree structure.
With the recent development of data centers, increase of use of Gigabit Ethernet, etc., a need has arisen to provide Ethernet functions in network devices. Also, a ring system is said to be suitable for efficiently connecting sites in cities or the like where a large communication traffic occurs.
To enable efficient use of a band in a large-capacity ring network such as Gigabit Ethernet, a method may be used in which a physical transmission path in the ring network is used by being divided into a plurality of logic topologies. FIG. 10 shows an example of such a method in which if the transmission rate of a transmission path 10 in ring form is 2.4 Gb/s in accordance with Synchronous Optical Network (SONET)/SDH (Synchronous Digital Hierarchy). In this example, the transmission path 10 can be divided into sixteen logic topologies 101 to 116 on a Synchronous Transfer Mode (STM)−1 (155 Mb/s) basis (shown in the figure as logic topologies 101 to 10n in general form).
A method has been adopted in which such a plurality of divided logic topologies 101 to 116 on a 155 Mb/s basis are individually used or several (m number) of them on the 155 Mb/s basis are combined (bundled) to be used as a logical transmission path of m×155 Mb/s.
Under these circumstances, a system may be constructed in such a manner that Ethernet functions are added to the above-described ring system. When the above-described ordinary spanning tree function is performed in this system, one of a plurality of node devices 1a to 1e constituting the ring system is set as a root bridge (indicated by R) of a spanning tree, as shown in FIG. 11.
An ideal system for effectively using a band in a ring network performs transfer using a shortest route as shown in FIG. 12. In the system shown in FIG. 11, there is no traffic through the transmission path between the node devices 1d and 1c and traffics are concentrated in the vicinity of the node device 1a (root bridge R). That is, the example of the system shown in FIG. 11 has a problem of traffic being doubled in the vicinity of the root bridge R in comparison with the ideal system shown in FIG. 12.